This invention relates to a fluid coupling for transmitting rotary torque.
A cooling fan, such as used for cooling an engine of a motor vehicle, is driven by the engine. However, when the rate of engine revolution is high, the fan provides more cooling air flow than is necessary. Therefore, a fluid coupling is mounted between the engine and the cooling fan, so that the fan is not rotated more than a prescribed rate of revolution when the rate of engine revolution is over a predetermined value. A conventional fluid coupling of this type uses fluid, such as silicone oil, and is so constructed that when the rate of engine revolution is low, it transmits rotary torque directly from an engine to a cooling fan. However, when the rate of engine revolution is higher than a predetermined value, the rotary resistance of the cooling fan increases over a predetermined value and the driven side (cooling fan) of the fluid coupling slips, and therefore, the conventional coupling does not transmit rotary torque higher than the predetermined rate. In such a conventional fluid coupling there are some disadvantages, such as, oil with high viscosity and stability is required. Because this type of coupling depends on the viscosity of the oil, the maximum rate of revolution of the driven side (cooling fan) is not maintained at a constant value, and the driven side (cooling fan) follows and rotates with the drive side when the rate of engine revolution is high.